arm: dts: mt7623: add pmic wrapper nodes to the mt7623 dtsi file

Add PMIC wrapper node to the mt7623.dtsi file which
is necessary for the control of PMIC from Mediatek.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>